1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory device, specifically to a nonvolatile semiconductor memory device to which a multi-layer metal process is applied.
2. Description of the Related Art
A Mask ROM (Read Only Memory) which is programmed by modifying a mask has been known as a nonvolatile semiconductor memory device. There are several programming methods for the Mask ROM including the following.
(1) Diffusion mask programming method in which a memory transistor is connected or disconnected to a bit line according to whether a diffusion layer is provided or not provided.
(2) Ion implantation mask programming method in which conduction of a memory transistor is altered according to whether ions are implanted or not implanted into a channel region of the memory transistor.
(3) Contact hole mask programming method in which a memory transistor is connected or disconnected to a bit line according to whether a contact hole is provided or not provided.
TAT (Turn Around Time) of a Mask ROM can be made shorter when a programming process step is closer to the final step in manufacturing the Mask ROM, since the programming process step is applied on the Mask ROM after receiving a purchase order from a customer in general. That is, a period of time from the receipt of the purchase order to delivery of the Mask ROM can be reduced.
Among the programming methods of the Mask ROM mentioned above, the diffusion mask programming method is least advantageous to reduce the TAT, because the diffusion process is in an early stage in the manufacturing process of the Mask ROM.
The ion implantation process can be performed at a later stage in manufacturing process of the Mask ROM, thus the TAT can be reduced with the ion implantation mask programming method. However, when the ion implantation mask programming method is adopted into the Mask ROM to which the multi-layer metal process is applied, implanting ions into channel regions of memory transistors requires implanting ions with acceleration energy high enough to penetrate stacked multi-layer insulating layers or implanting ions after etching the insulating layers to some extent so that ions can be implanted with relatively low acceleration energy, resulting in increased complexity of the manufacturing process.